Singer Engelbert's entertaining over? (6)
I believe the answer is:
treble
'singer' is the definition.
(I know that treble is a type of singer)
'engelbert's entertaining over?' is the wordplay.
'entertaining' says the answer is hidden in the clue.
'over?' says the letters should be written in reverse (I've seen 'over ' mean this).
'elbert' is hidden in the letters of 'engelberts'.
'elbert' backwards is 'TREBLE'.
(Other definitions for treble that I've seen before include "Singer; in three parts" , "Boy chorister" , "Soprano - multiply by three" , "bet?" , "Threefold; boy's voice" .)